Melanterite

Melanterite is a mineral form of hydrous iron(II) sulfate : FeSO4·7H2O. It is the iron analogue of the copper sulfate chalcanthite by loss of water which forms from the oxidation of primary sulfide minerals in the near-surface environment. It often occurs as a post mine encrustation on old underground mine and lignite and as a rare sublimate fumaroles Associated minerals include pisanite , pickeringite and other sulfate minerals.

It was first described in 1850.